The present invention is related to the cylinder group or groups provided with single-wire draw in the drying section of a paper machine or a paper finishing machine. Single-wire draw is known in the art to means such a mode of drawing and supporting the web in which the drying wire runs in a meandering manner from one row of cylinders to the other such that on the heated cylinders, the web enters into direct contact with the cylinder face and, on the leading cylinders or rolls placed between the drying cylinders, the web is placed at the side of the outside curve.
As is well known, moving faces induce air flows which move therealong with, and in a drying section this results in the drawback that an area of positive pressure is formed in the inlet nip between the drying wire and the leading cylinder. This pressure tends to detach the web from the drying wire, causing a case bag to be formed in the web. It is a further drawback that, when the drying wire and the web depart from the drying cylinder, occassionally the web attempts to follow the face of the drying cylinder, in which case a break or a wrinkle may be formed in the web.
The problems discussed above have been eliminated efficiently by means of the applicant's "UNO-RUN-BLOW-BOX" and "UNO-VAC".RTM. rolls, which are used as leading rolls between heated drying cylinders. With respect to the applicant's "UNO-RUN-BLOW-BOXes".RTM., reference is made to the applicant's FI Patents Nos. 65,460 and 69,332 (corresponding to U.S. Pat. Nos. 4,516,330 and 4,628,618, respectively). With respect to the "UNO-VAC"R rolls, reference is made to the applicant's FI Patents Nos. 881106, 893893 and 895928, of which the first one corresponds to U.S. Pat. Appl. 320,985. The above-mentioned U.S. Patents are hereby incorporated by reference.
Even though, in itself, by means of its suction effect, a "UNO-VAC".RTM. roll keeps the paper firmly in contact with the surrounding wire, it has, however, been found that at an inlet nip, the wedge of air arriving along with the wire and with the roll face produces a pressure impact through the wire onto the web, and in the web a convex bump bag is formed. Thus, the paper is detached from the wire at a distance of from about 50 to about 150 mm. This extra stretching is a detrimental phenomenon which may cause wrinkles or folds in the web. This detrimental phenomenon can be prevented by the use of these "BLOW BOXes".RTM., which operate by means of air supplied by a blower and in which the air discharged out of a nozzle slot evacuates air out of the nip.
Occassionally however, the requirement of constant pumping capacity and the highly sophisticated construction have been experienced as drawbacks of the "BLOW-BOXes".RTM.. Also, in some cases, the locating of the air pipes passing into the blow boxes is not entirely easy, because of the limited space available.